During cardiac surgery, a heart-lung machine is utilized. The heart-lung machine is used by being embedded in an extracorporeal circulation circuit and performs oxygenation of blood drained from a patient and filtering for elimination of foreign bodies, and the like. There are various types of heart-lung machines (also known as perfusion systems) depending on differences such as the type of gas exchange unit or oxygenator (i.e., artificial lung), the type of pump, the position where the pump is disposed, and the like. A heart-lung machine generally includes a reservoir (for venous blood), an artificial lung, a heat exchanger, a pump, and a plurality of tubes for connecting the components (for example, refer to US patent application publication US2009/0175762A1).
One type of pump adapted for use in the heart-lung machine is a centrifugal pump which delivers blood by utilizing centrifugal force occurring due to rotations of an impeller. Generally, the centrifugal pump includes a housing, wherein the impeller is rotatably disposed in the housing by a shaft which is provided at the center rotational axis of the impeller. A bearing on the housing rotatably supports the shaft. In order to reduce an occurrence of a thrombus (blood clot), a centrifugal pump may be used in which the bearing is formed as a pivot bearing.
Incidentally, in a case of the centrifugal pump employing the pivot bearing, a predetermined tightening (i.e., clamping) load is preferably applied to the shaft in the axial direction so that the shaft is rotatably supported in the housing. In this case, when the tightening load is too small, oscillations during rotations of the shaft may be increased so that hemolysis (damage to blood corpuscles) is more likely to occur. On the other hand, when the tightening load is too large, a thrombus is also more likely to occur. Therefore, it is preferable that the tightening load applied to the shaft and the bearing is set within a proper range.
Over long-term use of the centrifugal pump, the clamping load (tightening load) is continuously applied between the bearing and the housing. Therefore, there are cases where the depth of the recessed bearing surface of the bearing increases due to wear or the housing becomes warped. When a dimensional gap occurs between the members due to such deformation, the clamping load applied to the shaft and the bearing is deviated from the proper predetermined range. Accordingly, oscillations (shaft shaking) during rotations of the shaft increase so that hemolysis is more likely to occur.